The present invention relates to an optical disc drive which writes and/or reads signals to and/or from an optical disc by radiating a light to the latter, and more particularly to an optical disc drive adapted to transmit a light used to write and/or read signals to and/or from an optical disc through a polarization maintaining optical fiber.
Heretofore, optical discs such as ROM (read-only memory) type optical discs represented by CD (compact disc) and RAM (random-access memory) type optical discs represented by phase-change disc and magneto-optical disc, have widely been used as recording medium to store information including image, sound, character data, etc.
For such optical discs, it has been actively tried to increase the recording density for an increased storage capacity. Along with this trend, various attempts have been made to adapt the optical disc drives for successful write and/or read of signals to and/or from such higher recording density optical discs.
Recently, it has been proposed to apply the flying head technique already implemented in the hard disc drive to the optical disc drive in order to attain a higher recording density of the optical disc.
In this optical disc drive, an objective lens to converge a light on an optical disc is mounted on a slider to form a head and a light emitted from a light source is guided to the head through an optical fiber. In the optical disc drive, the head is scanned over the optical disc while being floated above the optical disc, and a spot of the light converged by the objective lens is radiated onto the optical disc to write and/or read signals to and/or from the optical disc.
An optical disc drive which writes and/or reads signals to and/or from an optical disc should desirably be compatible with all types of optical discs. However, when a magneto-optical disc is used in an optical disc drive which uses an optical fiber for transmission of a light, there will take place such problems as will be described in the following.
When the magneto-optical disc as a recording medium is used in the optical disc drive since signal detection is effected by the use of the Kerr effect that the polarization axes of the linearly polarized light are rotated under the effect of the magnetic field, it is necessary to transmit the light while keeping the light polarized. However, ordinary optical fibers show a random birefringence caused by a disturbance etc. having taken place during manufacture. Hence, a phase difference will take place between electric field vibration components of a light allowed to travel through the optical fiber and will cause a complex change in polarized state of the light.
The polarization maintaining optical fiber has already been known as an optical fiber capable of transmitting a light while maintaining the polarized state of the light. The polarization maintaining optical fiber has a waveguide structure having an advanced phase axis and retarded phase axis (two axes difference in refractive index) perpendicular to each other. In the waveguide structure of the optical fiber, a linearly polarized light incident along any one of the axes can be transmitted while the polarized state is being maintained.
However, even in case a light is guided through such a polarization maintaining optical fiber, when a magneto-optical disc is used as a recording medium with the conventional optical disc drive, the return light from the magneto-optical disc will be incident at a Kerr rotation angle upon the polarization maintaining optical fiber, and thus a phase difference will be caused between the field vibration components of the return light guided through the polarization maintaining optical fiber and will adversely affect the detection of signals.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing an optical disc drive compatible with all types of optical discs including magneto-optical discs.
The above object can be attained by providing an optical disc drive including means for rotating an optical disc, a light source to emit a light used for write and/or read of signals to and/or the optical disc, a head having installed therein an objective lens which converges the light emitted from the light source and being scanned over the optical disc being rotated by the rotating means to radiate the light converged by the objective lens to the optical disc, means for receiving a return light being a part of the light radiated from the head to the optical disc and which is reflected from the optical disc and detecting signals from the return light, and means for guiding the light emitted from the light source to the head and the return light from the optical disc to the signal detecting means.
In the above optical disc drive according to the present invention, the light transmitting means includes a polarization maintaining optical fiber and phase difference generating means to cancel out a phase difference between electric field vibration components generated when the light going towards the head or the return light going towards the signal detecting means is guided through the polarization maintaining optical fiber by the phase difference generated by the phase difference generating means or multiply the phase difference by an integer multiple of xcfx80.
In this optical disc drive, an installed optical disc is rotated by the disc rotating means, and a light used to write and/or read signals to and/or from the optical disc is emitted from the light source.
The light emitted from the light source is guided to the head by the light transmitting means including the polarization maintaining optical fiber and phase difference generating means. The head is scanned over the optical disc being rotated by the disc rotating means, and the light guided through the light transmitting means from the light source is converged by the objective lens and radiated to the optical disc.
A return light being a part of the light radiated from the head to the optical disc and reflected from the optical disc and which carries signals written on the optical disc, is guided by the light transmitting means to the signal detecting means. The signal detecting means thus receives the return light to detect the signals.
In the above optical disc drive according to the present invention, in case a magneto-optical disc from which signals are detected by the use of the Kerr effect is used as an optical disc, a return light from the magneto-optical disc will be incident in an off-axis manner upon the polarization maintaining optical fiber of the light transmitting means. Thus, in the return light guided through the polarization maintaining optical fiber, there will take place a phase difference between electric field vibration components depending upon the birefringence of the polarization maintaining optical fiber. In this optical disc drive, however, the phase difference between the field vibration components, having taken place in the light guided through the polarization maintaining optical fiber, is canceled out by a phase difference generated by the phase difference generating means or is multiplied by an integer multiple of xcfx80, so that the signals written on the magneto-optical disc can be detected properly.
That is, since it is possible to predict a distribution of refractive indices of the birefringence in the polarization maintaining optical fiber, magneto-optical signals can properly be detected by allowing the phase difference generating means to generate a predetermined phase difference in a light going towards the bead or a return light going towards the signal detecting means by means of the phase difference generating means correspondingly to a phase difference between field vibration components of the light, generated due to a birefringence of the polarization maintaining optical fiber and thus canceling out a phase difference between field vibration components, generated when the light is guided through the polarization maintaining optical fiber, or by multiplying the phase difference by an integer multiple of xcfx80 to suppress the influence of the birefringence of the polarization maintaining optical fiber.
In the above optical disc drive, the light transmitting means is formed from the polarization maintaining optical fiber and phase difference generating means on the basis of the above-mentioned principle to cancel out a phase difference between field vibration components, having taken place in a light guided through the polarization maintaining optical fiber, by a phase difference generated by the phase difference generating means or to multiply the phase difference by an integer multiple of xcfx80, thereby permitting to properly detect magneto-optical signals. Thus, the optical disc drive according to the present invention is compatible with all types of optical discs including magneto-optical discs.
Note that in the above optical disc drive, the phase difference generating means of the light transmitting means should desirably be formed from a combination of a pair of wedge-shaped birefringent crystals whose crystal axes are perpendicular to each other. By using the combination of a pair of the wedge-shaped birefringent crystals whose crystal axes are perpendicular to each other as the phase difference generating means, it is possible to simply and accurately adjust a phase difference generated by the phase difference generating means and also to effectively suppress the light guided through the phase difference generating means from incurring an optical axis offset.
Also the above object can be attained by providing an optical disc drive including means for rotating a plurality of optical discs together, a light source which emits a light for use to write and/or read signals to and/or from the plurality of optical discs, a plurality of heads provided correspondingly to the plurality of optical discs, each of which has installed thereon an objective lens to converge a light emitted from the light source and is canned over an appropriate optical disc being rotated by the disc rotating means to radiate the light converged by the objective lens to the optical disc, means for receiving a return light being a part of the light radiated to the optical disc from a corresponding one of the plurality of heads to detect signals carried by the light, and means for guiding the light emitted from the light source to each of the plurality of heads and the return light from the optical disc to the signal detecting means.
In the above optical disc drive according to the present invention, the light transmitting means includes a plurality of polarization maintaining optical fibers corresponding to the plurality of optical discs, optical path selecting means for making a selection between an optical path for the light emitted from the light source and a one for the return light going towards the signal detecting means, and means for generating a phase difference, a phase difference between field vibration components, having taken place when the light going towards the head or the return light going towards the signal detecting means goes through a one of the polarization maintaining optical fibers, selected by the optical path selecting means, being canceled out by a phase difference generated by the phase difference generating means or being multiplied by an integer multiple of xcfx80.
In this optical disc drive, a plurality of optical discs installed therein are rotated together by the disc rotating means. Also, a light for use to write and/or read signals to and/or from the optical discs is emitted from the light source.
The light emitted from the light source is guided to the head by the light transmitting means including the plurality of polarization maintaining optical fibers, optical path selecting means and phase difference generating means. At this time, the optical path selector selects an appropriate one of the plurality of polarization maintaining fibers for incidence of the light emitted from the light source upon the selected polarization maintaining optical fiber, whereby the light emitted from the light source will be guided to only a head corresponding to a specified one of the plurality of optical discs rotated together by the disc rotating means.
The head is canned over the specified optical disc and converges, by the objective lens, the light emitted from the light source and guided by the light transmitting means, for radiation to the optical disc.
A return light being a part of the light emitted from the slight source, radiated to the specified optical disc and reflected from the latter and which carries signals, is guided to the signal detecting means by the light transmitting means. The signal detecting means receives the return light and detects signals carried thereby.
In the above optical disc drive according to the present invention, in case a magneto-optical disc from which signals are detected by the use of the Kerr effect is used as an optical disc, a return light from the magneto-optical disc will be incident in an off-axis manner upon the polarization maintaining optical fiber of the light transmitting means. Thus, in the return light guided through the polarization maintaining optical fiber, there will take place a phase difference between electric field vibration components depending upon the birefringence of the polarization maintaining optical fiber. In this optical disc drive, however, the phase difference between the field vibration components, having taken place in the light guided through the polarization maintaining optical fiber, is canceled out by a phase difference generated by the phase difference generating means or is multiplied by an integer multiple of xcfx80, so that the signals written on the magneto-optical disc can be detected properly. Therefore, this optical disc drive is compatible with all types of optical discs including magneto-optical discs.
Also, in this optical disc drive, since an optical path is selected for a light emitted from the light source by the optical path selector of the light transmitting means to radiate the light to a specified one selected among the plurality of optical discs, and the signal detecting means receives the return light and detects signals carried by the return light, it is not necessary to provide more than one light source and signal detecting means correspondingly to the plurality of optical discs but the light source and signal detecting means can be used in common to all the optical discs. Therefore, the optical disc drive can be constructed from a reduced number of parts and thus can be produced with less costs.
Note that if a phase difference is caused to take place between field vibration components of a light going towards the head and return light going towards the signal detecting means when the optical path is selected for the light going to the head or return light going to the signal detecting means, it is desirable to adjust the phase difference to an integer multiple of xcfx80. For example, in case the optical path selector is arranged to change the optical path from one to another by reflecting an incident light, the phase difference taking place in the light can be adjusted to an integer multiple of xcfx80 by the use of a dielectric membrane to reflect the light.
Also, in this optical disc drive, the phase difference generating means of the light transmitting means should desirably be formed from a combination of a pair of wedge-shaped birefringent crystals whose crystal axes are perpendicular to each other. By using the combination of a pair of the wedge-shaped birefringent crystals whose crystal axes are perpendicular to each other as the phase difference generating means, it is possible to simply and accurately adjust a phase difference generated by the phase difference generating means and also to effectively suppress the light guided through the phase difference generating means from incurring an optical axis offset.
Also, in this optical disc drive, the phase difference generating means of the light transmitting means should desirably be provided in each of the plurality of heads corresponding to the plurality of optical discs. In this case, a phase difference generated by each of the phase difference generating means can be adjusted for each of the plurality of polarization maintaining optical fibers.
Also, in this optical disc drive, the phase difference generating means of the light transmitting means may be disposed in the optical path between the optical path selector and the light source and signal detecting means. In this case, the phase difference generating means can be used in common and thus the optical disc drive can be constructed from a reduced number of parts. In this case, however, the plurality of polarization maintaining optical fibers has to be equal in length to each other for a similar phase difference to take place in each of the polarization maintaining optical fibers.